Method for manufacturing an interdigitated back contact solar cell

ABSTRACT

A method for manufacturing an interdigitated back contact solar cell, comprising steps of: (a) providing a doped silicon substrate; (b) forming a first silicon dioxide layer on the front surface and the rear surface; (c) depositing a boron-containing doping paste on the first silicon dioxide layer of the rear surface in a first pattern; (d) heating the silicon substrate; (e) removing the first silicon dioxide layer; (f) forming a second silicon dioxide layer on the front surface and the rear surface; (g) depositing a phosphorus-containing doping paste on the second dioxide layer of the rear surface in a second pattern; (h) heating the silicon substrate; and (i) removing the second silicon dioxide layer from the silicon substrate, wherein the first pattern and the second pattern collectively form an interdigitated pattern.

FIELD OF THE INVENTION

This disclosure relates to an interdigitated back contact solar cellwhere localized boron diffused areas and phosphorus diffused areas areformed on the rear side.

TECHNICAL BACKGROUND OF THE INVENTION

One approach to overcome the limitations of a conventionalfront-contacted solar cell is to move both the p-n junction and the backsurface field (BSF) to the rear side of the solar cell. Thisarchitecture is referred to as the interdigitated back contact (IBC)solar cell. The concept of the IBC solar cell was first proposed byLammert and Schwartz [M. D. Lammert and R. J. Schwartz, “Theinterdigitated back contact solar cell: a silicon solar cell for use inconcentration,” IEEE Translations on Electron Devices, vol. 24, no. 4,pp. 337-342, 1977].

By moving both the p-n junction and BSF to the rear, all metallizationrequired to extract generated carriers is also moved to the rear side.The front, sunward side is therefore free from shading losses, enablinghigher short circuit current and thus efficiency. The front surface nolonger needs a heavily diffused layer optimized for metal contact, butinstead can be optimized to create a front surface field (FSF) whichminimizes recombination losses at the front surface thereby increasingthe open circuit voltage and thus efficiency. Another advantage ofhaving metal contacts on the rear side is that the metal geometry is nolonger constrained to minimize shading losses, allowing for wider metalwhich reduces resistive losses. Having all metal contacts on the backside also has the added benefit of simplifying integration of solarcells into a module.

Various methods have been suggested with regard to the manufacturingprocess of the IBC cell. Such methods are described in P. J. Verlinden,R. M. Swanson and R. A. Crane, Prog. Photovolt: Res. Appl. 2, 143-152(1994); F. Granek, “High-Efficiency Back-Contact Back-Junction SiliconSolar Cells,” PhD Thesis, Fraunhofer Institute (ISE), Freiburg, Germany(2009); D-H. Neuhaus and A. Munzer, “Review Article: Industrial SiliconWafer Solar Cells,” Advances in OptoElectronics, Volume 2007, Article ID24521, doi: 10.1155/2007/2451; US 2011/0003424; and US2010/0081264.

SUMMARY OF THE INVENTION

This invention describes a method of forming a high efficiencyinterdigitated back contact (IBC) solar cell employing localized boronand phosphorus diffused areas on the rear side achieved using a dopingpaste(s) and an oxide barrier layer. Introducing an oxide layer prior toprinting patterned doping pastes allows for boron and phosphorusdiffusion into the underlying wafer and at the same time eliminatesunintended doping, referred to as autodoping, onto neighboring siliconsurfaces.

An aspect of the invention is a method for manufacturing aninterdigitated back contact solar cell, comprising steps of: (a)providing a doped silicon substrate, the substrate comprising a front,sunward facing, surface and a rear surface; (b) forming a first silicondioxide layer on the front surface and the rear surface; (c) depositinga boron-containing doping paste on the first silicon dioxide layer ofthe rear surface in a first pattern, wherein the boron-containing pastecomprises a boron compound and a solvent; (d) heating the siliconsubstrate in a first ambient to a first temperature and for a first timeperiod in order to locally diffuse boron into the rear surface of thesilicon substrate, thereby forming a p+ region on the rear surface ofthe silicon substrate; (e) removing the first silicon dioxide layer fromthe silicon substrate; (f) forming a second silicon dioxide layer on thefront surface and the rear surface; (g) depositing aphosphorus-containing doping paste on the second dioxide layer of therear surface in a second pattern, wherein the phosphorus-containingdoping paste comprising a phosphorus compound and a solvent; (h) heatingthe silicon substrate in a second ambient to a second temperature andfor a second time period in order to locally diffuse phosphorus into therear surface of the silicon substrate, thereby forming an n+ region onthe rear surface of the silicon substrate; and (i) removing the secondsilicon dioxide layer from the silicon substrate, wherein the firstpattern and the second pattern collectively form an interdigitatedpattern.

Another aspect of the invention is a method for manufacturing aninterdigitated back contact solar cell, comprising steps of: (a)providing a doped silicon substrate, the substrate comprising a front,sunward facing, surface and a rear surface; (h) doping the rear surfaceof the substrate homogeneously with boron in a first pattern, therebyforming a p+ region on the rear surface of the silicon substrate,wherein the first pattern is a blanket pattern; (c) forming a silicondioxide layer on the front surface and the rear surface; (d) depositinga phosphorus-containing doping paste on the rear surface in a secondpattern, wherein the phosphorus-containing doping paste comprising aphosphorus compound and a solvent; (e) heating the silicon substrate inan ambient to a temperature and for a time period in order to locallydiffuse phosphorus into the rear surface of the silicon substrate,thereby forming an n+ region on the rear surface of the siliconsubstrate through the second pattern, wherein the p+ region and the n+region on the rear surface collectively form an interdigitated pattern;and (f) removing the second silicon dioxide layer from the siliconsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an interdigitated back-contact (IBC)solar cell.

FIGS. 2A-2B schematically show autodoping associated with a patterneddoping source subjected to a thermal drive-in process. FIG. 2A shows aconfiguration where two silicon wafers are vertically parallel to eachother. FIG. 2B shows the effects of a high temperature treatment on thewafers shown in FIG. 2A.

FIGS. 3A-3B schematically show a method for achieving localized dopingfree from autodoping using a patterned doping source and a barrier SiO₂layer. FIG. 3A shows two silicon wafers (301) and (303) with SiO₂ layer(310) in the same configuration as the wafers in FIG. 2A. FIG. 3B showsthe effects of a high temperature treatment on the wafers shown in FIG.3A.

FIGS. 4A-4R show a process for incorporating the doping-through-oxideapproach to fabricate an IBC solar cell. FIG. 4A shows the texturizingstep. FIG. 4B shows the phosphorus diffusion step. FIG. 4C shows theremoval step of the rear side phosphorus diffused layer. FIG. 4D showsthe oxidation step. FIG. 4E shows printing step of the boron paste. FIG.4F shows the boron drive-in step. FIG. 4G shows the oxide layerstripping step. FIG. 4H shows the oxidation step. FIG. 4I shows theprinting step of the phosphorus paste. FIG. 4J shows the phosphorusdrive-in step. FIG. 4K shows the oxide layer stripping step. FIG. 4Lshows the passivation step. FIG. 4M shows the metallization(electrode-forming) step. FIG. 4N shows a planview of two boron emittersand two phosphorus BSF regions. FIG. 4O shows a pattern of the boronemitter metal contact. FIG. 4P shows a pattern of the BSF metal contact.FIG. 4Q shows a pattern of the insulator layer. FIG. 4R shows a patternof the metal busbars.

FIGS. 5A-5M show an alternative process for incorporating thedoping-through-oxide approach to fabricate an IBC solar cell. FIG. 5Ashows the texturizing step. FIG. 5B shows the phosphorus diffusion step.FIG. 5C shows the removal step of the rear side phosphorus diffusedlayer. FIG. 5D shows the oxidation step. FIG. 5E shows the blanketdeposition step of the boron paste. FIG. 5F shows the boron drive-instep. FIG. 5G shows the oxide layer stripping step. FIG. 5H shows theoxidation step. FIG. 5I shows the printing step of the phosphorus paste.FIG. 5J shows the phosphorus drive-in step. FIG. 5K shows the oxidelayer stripping step. FIG. 5L, shows the passivation step. FIG. 5M showsthe metallization (electrode-forming) step.

FIG. 6 shows a schematic view of the interdigitated pattern inExperiment 1.

FIGS. 7A-7C show SIMS profiles. FIG. 7A shows the boron and phosphorusSIMS profiles in the center of the boron paste printed areacorresponding to the emitter region. FIG. 7B shows the boron andphosphorus SIMS profiles in the far edge of the boron printed region.FIG. 7C shows the boron and phosphorus SIMS profiles in the center ofthe non-printed region corresponding to the gap area. FIG. 7D shows theboron and phosphorus SIMS profiles in the center of the phosphorus pasteprinted area corresponding to the BSF region.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings, in the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

A schematic view of an interdigitated back-contact (IBC) solar cell isshown in FIG. 1. The IBC solar cell of FIG. 1 is an embodiment and thescope of the present invention is not limited to the specific type ofthe IBC solar cell. For example, a boron doped silicon substrate can beused instead of a phosphorus doped silicon substrate. In the embodimentof FIG. 1, a phosphorus doped silicon substrate (101) is used as theabsorber. A boron emitter (102) is formed on the rear side of the cellto create the p-n junction required to separate carriers. A phosphorusBSF (103) is also formed on the rear side of the solar cell. The boronand phosphorus doped regions are formed in interdigitated patterns. AFSF (104) is formed on the front, sunward side of the solar cell,typically employing a phosphorus diffusion. The FSF repels minoritycarriers from the front surface thus minimizing recombination losses.The FSF also forms a current path for majority carriers to reach the BSFregion and BSF contact.

The front and rear surfaces are coated with a passivation film, shown as(105) and (106), respectively. Such passivation film may comprise atleast one of silicon nitride, aluminum oxide and silicon oxide. In someembodiments, the passivation film may be a stack structure comprisingmultiple, i.e. two or more layers. Such stack structure may include atleast one layer comprising silicon nitride and at least one layercomprising one of silicon oxide and aluminum oxide. Boron emitter metalcontacts (107) and BSF metal contacts (108) are formed on the rear sidein order to extract generated carriers.

One approach that reduces the number of thermal steps is to screen printboron and/or phosphorus doping pastes in the required interdigitatedpattern followed by a thermal drive-in process to transfer dopant atomsfrom the pastes into the underlying silicon substrate.

Achieving patterned and localized boron and phosphorus doping on asilicon wafer using boron-containing and phosphorus-containing dopingpastes is made difficult by the tendency of these pastes to outgasduring the thermal treatment required to diffuse dopants into the wafer.In order to diffuse boron or phosphorus from the doping pastes into theunderlying silicon substrate, a thermal treatment at temperatures above800° C. is typically used. At these elevated temperatures gaseousboron-containing and phosphorus-containing species are created fromboron-containing and phosphorus-containing pastes, respectively. Thesegaseous species transfer onto nearby silicon surfaces, causing aspreading of dopant outside of the intended printed areas.

Gas-phase distribution of an initially patterned dopant region is calledautodoping and degrades the pattern of the doped regions. Gas phasedistribution and autodoping from phosphorus-containing doping pastes hasbeen documented, for example, in [A. Mouhoub, B. Benyahia, B. Mahmoudi,and A. Mougas, “Selective Emitters for Screen Printed MulticrystallineSilicon Solar Cells,” Rev. Energ. Ren.: ICPWE (2003) 83-86; L. Debarge,M. Schott, J. C. Muller, and R. Monna, “Selective emitter formation witha single screen-printed p-doped paste deposition using out-diffusion inan RTP-step,” Sol. Mat. Sol. Cells 74 (2002) 71-75; and M. Edwards, J.Bocking, J. E. Cotter, and N. Bennet, “Screen-Print Selective Diffusionsfor High Efficiency Industrial Silicon Solar Cells,” Prog. Photovolt:Res. Appl. 16: 31-45 (2008)]. Boron autodoping has been observed fromheavily doped epitaxial silicon layers which act as doping sourceregions for neighboring silicon wafers [M. J. Binns, S. Kommu, M. R.Seacrist, R. W. Standley, R. Wise, D. J. Myers, D. Tisserand, and D.Doyle, “The Control of Boron Autodoping during Device Processing forp/p+ Epi Wafers with no Back-Surface Oxide Seal,” Proceedings from9^(th) Int. Symp. Silicon Materials Science & Technology (2002)].

FIG. 2 schematically shows the autodoping associated with a patterneddoping source subjected to a thermal drive-in process. FIG. 3schematically shows a method for achieving localized doping free fromautodoping using a patterned doping source and a barrier SiO₂ layer.FIG. 2A shows two silicon wafers (201) and (203) vertically parallel toeach other, representing the typical orientation used when processingwafers inside of a quartz tube furnace. The rear surface (202) of wafer(201) is adjacent to the front side (204) of wafer (203). Also shown inFIG. 2A are patterned doping source regions (205) which can be eitherboron or phosphorus paste. When subjected to high temperature treatment,the doping source regions are intended to create localized doping of theunderlying silicon substrate, i.e. dopant atoms are to be introducedinto the substrate only in areas directly underneath the doping source.

FIG. 2B shows the effects of a high temperature treatment on the wafersshown in FIG. 2A. Wafers (201) and (203) are subjected to a hightemperature drive-in step, typically 800° C. or higher, in order todrive dopant atoms from the doping pastes into the underlying siliconsubstrate forming doped areas (207) directly beneath the doping sources.At these elevated temperatures, dopant species (206) transfer from thedoping source regions (205) onto the rear surface (202) of theunderlying wafer (201) as well as onto the front side (204) of theneighboring wafer (203). The transfer of dopant species (206) results inunintended doping, referred to as autodoping, creating a doped layer(208) on the rear side (202) of the underlying wafer (201) as well ascreating a doped layer (209) on the front side of the neighboring wafer(203). Autodoping results in the loss of the localized doping.

Maintaining localized doping and eliminating autodoping can be achievedby introducing a protective SiO₂ layer on the silicon wafer surfaceprior to applying a patterned doping source layer.

FIG. 3 shows two silicon wafers (301) and (303) in the sameconfiguration as the wafers in FIG. 2A, but with each wafer having allsurfaces covered with a SiO₂ layer (310). The rear surface (302) ofwafer (301) is adjacent to the front side (304) of wafer (303). Alsoshown in FIG. 3A are patterned doping source regions (305) which areseparated from the rear surface (302) by the SiO₂ layer (310). Thedoping sources (305) can be either boron or phosphorus pastes.

FIG. 3B shows the effects of a high temperature treatment on the wafersshown in FIG. 3A. Wafers (301) and (303) are subjected to a hightemperature drive-in step, typically 800° C. or higher. The dopingsource regions (labeled as (305) in FIG. 3A) locally react with theunderlying SiO₂ creating new doping source layers (305′). The dopingsource layers (305′) make contact with the underlying silicon wafersurface. Dopant atoms are driven from the doping source layers (305′)into the underlying wafer, creating localized doped areas (307).

At these elevated temperatures dopant species (306) transfer from thedoping source regions (305′) onto the SiO₂ layer (310) covering the rearsurface (302) of the underlying wafer (301) as well as onto the SiO₂layer (310) covering the front side (304) of the neighboring wafer(303). The SiO₂ layer (310) acts as a barrier for areas not covered withthe dopant source layer (305′), preventing dopant species (306) fromreaching the rear surface (302) of the underlying wafer (301) as well asonto the front side (304) of the neighboring wafer (303). The SiO₂ layer(310) enables localized doping and eliminates autodoping.

An embodiment for incorporating the doping-through-oxide approach tofabricate an IBC solar cell is shown in FIGS. 4A-4R.

The fabrication process may begin with a textured n-type silicon wafer(401) as shown in FIG. 4A. The silicon wafer is then subjected to aphosphorus diffusion process, like a. POCl₃ diffusion process performedin a heated quartz tube furnace in a POCl₃, N₂ and O₂ ambient, formingan n+ layer (402) near the wafer surface as shown in FIG. 4B. Residualsurface phosphosilicate glass from the phosphorus diffusion can beremoved by submerging the wafer in a dilute HF solution.

The rear side phosphorus diffused layer can be removed using asingle-sided wet chemistry etch process such as using a hydrofluoricacid, nitric acid, and sulfuric acid mixture, leaving behind an n-typerear surface (403) as shown in FIG. 4C.

Front side phosphorus diffusion can also be achieved using other methodsincluding but not limited to: phosphoric acid spray process followed bya thermal drive-in step, front side PSG deposition using a chemicalvapor deposition system followed by a thermal drive-in step, or an ionimplantation process.

The wafer is then subjected to an oxidation process which grows a SiO₂layer (404) on the surface of the wafer as shown in FIG. 4D. A thermaloxidation process may be used which typically employs a heated quartztube furnace with an oxidizing ambient. This high temperature oxidationstep will at the same time drive the phosphorus atoms in the existingfront side n+ layer (shown as (402) in FIG. 4C) deeper into the wafer,thus creating a modified n+ layer shown as (402′) in FIG. 4D. In theevent a boron-doped substrate is used as the substrate (401), the dopedlayer (402 in FIG. 4C), which is doped with the same dopant as thedopant of the substrate (401), can be a p+ layer.

Alternatively, the oxide layer (404) may be formed by other methods suchas (i) thermal oxidation, (ii) depositing a liquid compositioncomprising silicon dioxide particles (e.g. printing, spraying, dippingetc.), (iii) chemical vapor deposition, (iv) chemical oxidation, (v)steam oxidation, (vi) printing and oxidizing a composition comprising ofsilicon particles, and (vii) growing and oxidizing a porous siliconlayer. In the cases of growing a chemical oxide or growing and oxidizinga porous silicon layer, the top heavily doped portion (closest to thewafer surface) of the existing front side n+ layer (shown as (402) inFIG. 4C) may be etched away thus creating a modified n+ layer similar tothat achieved using a thermal drive-in process (shown as (402′) in FIG.4D). The thickness of the SiO₂ layer is 10 nm to 100 in an embodiment.

For the next step, shown in FIG. 4E, a boron-containing paste is screenprinted onto the oxidized rear surface followed by a low temperaturebake in order to drive off solvents from the paste to form a patternedboron-containing paste area (405). Other methods can be used fordepositing the pastes instead of screen printing. The boron print willform the boron emitter after a subsequent thermal drive-in step.

The oxidized rear surface does not contain any via or any through holethat enables the direct contact of the pastes with the rear surface(403) of the substrate in an embodiment. As illustrated in FIG. 4E, theboron-containing paste is deposited on the SiO₂ layer and the borondiffuses into the silicon wafer in the following drive-in process.

The next step is a high temperature drive-in process. The process isperformed in a quartz tube furnace in an N₂ ambient in an embodiment.During this high temperature step the boron paste locally reacts withoxide layer forming new boron source layers (405′) which reach theunderlying silicon wafer and locally dope to form the boron emitter(406) as shown in FIG. 4F.

The temperature of the drive-in process is 850° C. to 1000° C. in anembodiment. The time period of the drive-in process is 20 minutes to 2hours in an embodiment.

This high temperature drive-in step can at the same time drive thephosphorus atoms in the existing front side n+ layer (shown as (402′) inFIGS. 4D and 4E) deeper into the wafer thus creating a modified n+ layershown as (402″) in FIG. 4F in an embodiment. The next step is to removethe oxide layer and residual paste layers exposing the n+ layer (402″)on the front side and the boron emitter (406) on the rear side as shownin FIG. 4G. The oxide layer can be removed by submerging the wafers in adilute HF solution.

Boron diffusion processes are known to sometimes form an undesirableboron-rich layer (BRL) at the wafer surface. This BRL layer has beenidentified as boron silicide (SiB₆). In an embodiment, a BRL can beremoved by subjecting the wafers to an oxidizing ambient between 600° C.and 800° C. The oxidized BRL layer can then be removed by submerging thewafers in a dilute HF solution.

The wafer is then subjected to another oxidation process which grows aSiO₂ layer (404) on the surface of the wafer as shown in FIG. 4H. Athermal oxidation process may be used which typically employs a heatedquartz tube furnace with an oxidizing ambient. This high temperatureoxidation step will at the same time further drive the phosphorus atomsin the existing front side n+ layer (shown as (402″) in FIG. 4G) deeperinto the wafer, thus creating a modified n+ layer shown as (402′″) inFIG. 4H. Also, during the thermal oxidation the boron emitter (shown as(406) in FIG. 4G) will have its profile modified by both segregation ofboron atoms into the oxide layer and by the high temperature of theprocess. These two effects will create a modified boron emitter (406′)with a lower surface concentration and a deeper profile.

Alternatively, the oxide layer (404) may be formed by other methods suchas (i) thermal oxidation, (ii) depositing a liquid compositioncomprising silicon dioxide particles (e.g. printing, spraying, dippingetc.), (iii) chemical vapor deposition, (iv) chemical oxidation, (v)steam oxidation, (vi) printing and oxidizing a composition comprising ofsilicon particles, and (vii) growing and oxidizing a porous siliconlayer. The thickness of the SiO₂ layer is 10 nm to 100 in an embodiment.

For the next step, shown in FIG. 4I, a phosphorus-containing paste isscreen printed onto the oxidized rear surface followed by a lowtemperature bake in order to drive off solvents from the paste to form apatterned phosphorus-containing paste area (407).

The oxidized rear surface does not contain any via or any through holethat enables the direct contact of the pastes with the rear surface(403) of the substrate in an embodiment. As illustrated in FIG. 4I, thephosphorus-containing paste is deposited on the SiO₂ layer and thephosphorus diffuses into the silicon wafer in the following drive-inprocess.

The next step is a high temperature drive-in process. The process isperformed in a quartz tube furnace in an N₂ ambient in an embodiment.During this high temperature step the phosphorus paste locally reactswith oxide layer forming new phosphorus source layers (407′) which reachthe underlying silicon wafer and locally dope to form the phosphorus BSF(408) as shown in FIG. 4J.

The temperature of the drive-in process is 850° C. to 1000° C. in anembodiment. The time period of the drive-in process is 20 minutes to 2hours in an embodiment.

This high temperature drive-in step will at the same time drive thephosphorus atoms in the existing front side n+ layer (shown as (402′″)in FIGS. 4H and 4I) deeper into the wafer thus creating a yet furthermodified n+ layer shown as (402 _(iv)) in FIG. 4J in an embodiment. Thismodified n+ layer (402 ^(iv)) is the FSF. Also during this hightemperature step the boron atoms in the existing emitter regions (shownas 406′ in FIG. 4I) are driven deeper into the wafer creating a modifiedboron emitter regions shown as (406″) in FIG. 4J.

The next step is to remove the oxide layer and residual paste layersexposing the FSF (402 ^(iv)) on the front side, the boron emitter (406″)and the phosphorus BSF (408) on the rear side as shown in FIG. 4K. Theoxide layer can be removed by submerging the wafers in a dilute HFsolution. The final interdigitated doped pattern on the rear sideconsists of three regions: the boron emitter (406″), the phosphorus BSF(408) and the non-doped regions commonly referred to as the gap,indicated as (409) in FIG. 4K. In an embodiment, the gap region can beeliminated.

The front and rear surfaces are subsequently coated with passivationlayers (410) and (411), respectively, as shown in FIG. 4L. Suchpassivation films may comprise at least one of silicon nitride, aluminumoxide and silicon oxide. In some embodiments, the passivation film maybe a stack structure comprising multiple, i.e. two or more layers. Suchstack structure may include at least one layer comprising siliconnitride and at least one layer comprising one of silicon oxide andaluminum oxide. The passivation film may be prepared using a number oftechniques. In some embodiments, the passivation film may be depositedusing a chemical vapor deposition technique, such as plasma enhancedchemical vapor deposition (PECVD). Passivation films deposited on theback and front surfaces of the substrate may be the same or different.In some embodiments, a passivation film on the back surface of thesubstrate may be thicker than the one on the front surface of thesubstrate, while in some other embodiments, a passivation film on theback surface of the substrate may be thinner than the one on the frontsurface of the substrate. A thickness of the passivation film may vary.In some embodiments, such thickness may be from 30 nm to 300 nm or from50 nm to 250 nm or any value or subrange within these ranges.

The next step is to apply boron emitter metal con acts (412) andphosphorus BSF contacts (413), as shown in FIG. 4M. A process forforming the metal contacts (412, 413) is described below. However, thescope of the invention is not limited to the particular embodiment.

In one embodiment two different metal compositions are used for theemitter contact and the BSF contact. In another embodiment, the samemetal composition is used for both the emitter contact and the BSFcontact. In one embodiment, two metal compositions are depositedsequentially. In another embodiment, two metal compositions aredeposited simultaneously. These metal contacts are deposited as patternsconsisting of dots and/or lines. In order to describe the subsequentsteps, FIGS. 4N-4R employ a two dimensional plane view of the rear sideof the substrate.

FIG. 4N shows a series of two boron emitters indicated as (420) and aseries of two phosphorus BSF regions indicated as (421). The boronemitter metal contact pattern is shown in FIG. 4O as tines (422). Thenext step is to deposit patterned BSF metal line contacts (423) as shownin FIG. 4P. In an embodiment, the contact for the phosphorus diffusedregion is a silver paste, containing organics for printability andtraces of other metals or glass frits for etching SiN during the firingprocess. DuPont PV17x and DuPont PV16A are example products used forthis application. In an embodiment, the contact for the boron diffusedregions is a silver paste, containing over 80% or more silver powder byweight. The precise composition of the non-silver additives may bedifferent from the phosphorus diffusion contact paste. DuPont PV3N1 isan example of the product used for this application. The conductorlayers are fired at temperatures above 700° C. in a belt furnace to burnoff the organic components in an embodiment, density the silver line toincrease conductivity, and activate the frits which etch the SiN.

The next step is to deposit a patterned insulator layer (424) whichpartly overlaps the line segments of both the emitter contacts (422) andBSF contacts (423) as shown in FIG. 4Q. The insulator regions will allowmetal busbars of opposite polarity to that of the underlying dopedregion to traverse overtop these regions and avoid an electrical shuntpath and thus allow carriers to be extracted from the solar cell. In anembodiment, the dielectric layer is composed of ceramic or oxidepowders. In an embodiment, the dielectric layer is formed by screenprinting a paste comprising ceramic powders or oxide powders and anorganic vehicle. The organic vehicle may comprise a solvent and anorganic binder for adjusting the printability. The paste melts andreflows at temperatures equal to or lower than the belt-furnace firingtemperature to encapsulate the underlying conductor in an embodiment.There are many suitable compositions: compounds of boron, lead, andtitanium and can be mixed with silica to achieve this. In contrast tothe conductors, the glass materials in the dielectric paste are unableto etch SiN in an embodiment. DuPont 5415A is an example of a productused for this application. As an alternative to this ceramic or glassinsulator material, a lower temperature polymer material could be used,such as DuPont 5018. Low temperature insulators cannot be firedsimultaneously with the silver conductors.

The next step is to deposit metal busbars (425, 426) as shown in FIG.4R. The same metal is used and therefore all regions can be depositedsimultaneously in an embodiment. The emitter metal busbar (425)electrically connects all emitter metal lingers (422). The BSF metalbusbar (426) electrically connects all BSF metal fingers (423). In anembodiment, the busbar metal is a silver paste comprising of mostlysilver, with some organic additives. DuPont 5025 is an example productused for this application. Other, non-silver based metal pastes, can beused for the busbar. Depending on the choice of dielectric material, thebusbar may be fired at high temperature like the contact conductors, orat low temperature like the insulator.

Various modifications can be made in the above method. In an embodiment,the phosphorus-containing paste is printed on the rear side of the solarcell after the boron emitter is formed. In contrast, theboron-containing paste may be printed on the rear side of the solar cellafter the phosphorus BSF is formed.

Other modifications and improvements can be referred to. For example,patent applications such as U.S. patent application Ser. No. 13/602,919filed on Sep. 4, 2012, U.S. patent application Ser. No. 13/795,191 filedon Mar. 6, 2013, U.S. patent application Ser. No. 13/661,515 filed onOct. 26, 2012, US patent publication US20120145967 and US patentpublication US20120280183 are incorporated by reference in theirentirety to the same extent as if they were individually incorporated byreference. In an embodiment, p-type silicon wafer is used instead ofn-type silicon wafer. Various modifications such as interchanging thePOCl₃ diffusion process with a BBr₃ diffusion process can be employedfor p-type silicon wafers.

Another embodiment for incorporating the doping-through-oxide approachto fabricate an IBC solar cell is shown in FIGS. 5A-M.

The fabrication process may begin with a textured n-type silicon wafer(501) as shown in FIG. 5A. The silicon wafer is then subjected to aphosphorus diffusion process, like a POCl₃ diffusion process performedin a heated quartz tube furnace in a POCl₃, N₂ and O₂ ambient, formingan n+ layer (502) near the wafer surface as shown in FIG. 5B. Residualsurface phosphosilicate glass from the phosphorus diffusion can beremoved by submerging the wafer in a dilute HF solution.

The rear side phosphorus diffused layer can be removed using asingle-sided wet chemistry etch process using a hydrofluoric acid,nitric acid, and sulfuric acid mixture, leaving behind an n-type rearsurface (503) as shown in FIG. 5C. Front side phosphorus diffusion canalso be achieved using other methods including but not limited to:phosphoric acid spray process followed by a thermal drive-in step, frontside PSG deposition using a chemical vapor deposition system followed bya thermal drive-in step, or an ion implantation process.

The wafer is then subjected to an oxidation process which grows a SiO₂layer (504) on the surface of the wafer as shown in FIG. 5D. A thermaloxidation process may be used which typically employs a heated quartztube furnace with an oxidizing ambient. This high temperature oxidationstep will at the same time drive the phosphorus atoms in the existingfront side n+ layer (shown as (502) in FIG. 5C) deeper into the wafer,thus creating a modified n+ layer shown as (502′) in FIG. 5D. In theevent a boron-doped substrate is used as the substrate (501), the dopedlayer (502 in FIG. 5C), which is doped with the same dopant as thedopant of the substrate (501), can be a p+ layer.

Alternatively, the oxide layer (504) may be formed by other methods suchas (i) thermal oxidation, (ii) depositing a liquid compositioncomprising silicon dioxide particles (e.g. printing, spraying, dippingetc.), (iii) chemical vapor deposition, (iv) chemical oxidation, (v)steam oxidation, (vi) printing and oxidizing a composition comprising ofsilicon particles, and (vii) growing and oxidizing a porous siliconlayer. The thickness of the SiO₂ layer is 10 nm to 100 nm in anembodiment.

For the next step, shown in FIG. 5E, a boron-containing paste is screenprinted onto the oxidized rear surface in an unpatterned, blanket layer(505) followed by a low temperature bake in order to drive off solventsfrom the paste. The boron print will form a blanket pattern which aftera thermal drive-in process will create a homogeneous boron emitter.

The oxidized rear surface does not contain any via or any through holethat enables the direct contact of the pastes with the rear surface(503) of the substrate in an embodiment. As illustrated in FIG. 5E, theboron-containing paste is deposited on the SiO₂ layer and the borondiffuses into the silicon wafer in the following drive-in process.

The next step is a high temperature drive-in process. The process isperformed in a quartz tube furnace in an N₂ ambient in an embodiment.During this high temperature step the boron paste reacts with the oxidelayer forming new boron source layer (505′) which reaches the underlyingsilicon wafer and locally dopes to form the boron emitter (506) as shownin FIG. 5F.

The temperature of the drive-in process is 850° C. to 1000° C. in anembodiment. The time period of the drive-in process is 20 minutes to 2hours in an embodiment.

This high temperature drive-in step can at the same time drive thephosphorus atoms in the existing front side n+ layer (shown as (502′) inFIGS. 5D and 5E) deeper into the wafer thus creating a modified n+ layershown as (502″) in FIG. 5F in an embodiment.

The next step is to remove the oxide layer and residual paste layersexposing the n+ layer (502″) on the front side and the boron emitter(506) on the rear side as shown in FIG. 5G. The oxide layer can beremoved by submerging the wafers in a dilute HF solution.

The homogenous boron emitter can be created using different methodsincluding, but not limited to: ion implantation, BBr₃ or BCl₃ diffusionused in conjunction with a barrier layer on the front side of the wafer,and depositing a single-side BSG layer using a chemical vapor depositiontool followed by a high temperature drive-in step.

Boron diffusion processes are known to sometime form an undesirableboron-rich layer (BRL) at the wafer surface. This BRL layer has beenidentified as boron suicide (SiB₆). In an embodiment a BRL can beremoved by subjecting the wafers to an oxidizing ambient between 600° C.and 800° C. The oxidized BRL layer can then be removed by submerging thewafers in a dilute HF solution.

The wafer is then subjected to another oxidation process which grows aSiO₂ layer (504) on the surface of the wafer as shown in FIG. 5H. Athermal oxidation process may be used which typically employs a heatedquartz tube furnace with an oxidizing ambient. This high temperatureoxidation step will at the same time further drive the phosphorus atomsin the existing front side n+ layer (shown as (502″) in FIG. 5G) deeperinto the wafer, thus creating a modified n+ layer shown as (502′″) inFIG. 5H. Also during the thermal oxidation the boron emitter (shown as(506) in FIG. 5G) will have its profile modified by both segregation ofboron atoms into the oxide layer and by the high temperature of theprocess. These two effects will create a modified boron emitter (506′)with a lower surface concentration and a deeper profile.

Alternatively, the oxide layer (504) may be formed by other methods suchas (i) thermal oxidation, (ii) depositing a liquid compositioncomprising silicon dioxide particles (e.g. printing, spraying, dippingetc.), (iii) chemical vapor deposition, (iv) chemical oxidation, (v)steam oxidation, (vi) printing and oxidizing a composition comprising ofsilicon particles, and (vii) growing and oxidizing a porous siliconlayer. The thickness of the SiO₂ layer is 10 nm to 100 in an embodiment.

For the next step, shown in FIG. 5I, a phosphorus-containing paste isscreen printed onto the oxidized rear surface followed by a lowtemperature bake in order to drive off solvents from the paste to form apatterned phosphorus-containing paste area (507).

The oxidized rear surface does not contain any via or any through holethat enables the direct contact of the pastes with the rear surface(503) of the substrate in an embodiment. As illustrated in FIG. 5I, thephosphorus-containing paste is deposited on the SiO₂ layer and thephosphorus diffuses into the silicon wafer in the following drive-inprocess.

The next step is a high temperature drive-in process. The process isperformed in a quartz tube furnace in an N₂ ambient in an embodiment.During this high temperature step the phosphorus paste locally reactswith oxide layer forming new phosphorus source layers (507′) which reachthe underlying silicon wafer and locally dope to form the phosphorus BSF(508) as shown in FIG. 5J. During this step the phosphorous diffusionwill compensate the existing boron emitter (506′) in the regions wherephosphorous paste (507) is printed. The temperature of the drive-inprocess is 850° C. to 1000° C. in an embodiment. The time period of thedrive-in process is 20 minutes to 2 hours in an embodiment.

This high temperature drive-in step can at the same time drive thephosphorus atoms in the existing front side n+ layer (shown as (502′″)in FIGS. 5H and 5I) deeper into the wafer thus creating a yet furthermodified n+ layer shown as (502 ^(iv)) in FIG. 5J in an embodiment. Thismodified n+ layer (502 ^(iv)) is the FSF. Also during this hightemperature step the boron atoms in the existing emitter regions (shownas 506′ in FIG. 5I) are driven deeper into the wafer creating a modifiedboron emitter regions shown as (506″) in FIG. 5J. It should be notedthat the phosphorus doping in the BSF regions (508) needs to be heavyenough to overcompensate the underlying boron doping (506″).

The next step is to remove the oxide layer and residual paste layersexposing the FSF (502 ^(iv)) on the front side, the boron emitter (506″)and the phosphorus BSF (508) on the rear side as shown in FIG. 5K. Theoxide layer can be removed by submerging the wafers in a dilute HFsolution.

The front and rear surfaces are subsequently coated with passivationlayers (509) and (510), respectively, as shown in FIG. 5L. Suchpassivation film may comprise at least one of silicon nitride, aluminumoxide and silicon oxide. In some embodiments, the passivation film maybe a stack structure comprising multiple, i.e. two or more layers. Suchstack structure may include at least one layer comprising siliconnitride and at least one layer comprising one of silicon oxide andaluminum oxide. The passivation film may be prepared using a number oftechniques. In some embodiments, the passivation film may be depositedusing a chemical vapor deposition technique, such as plasma enhancedchemical vapor deposition (PECVD). Passivation films deposited on theback and front surfaces of the substrate may be the same or different.In some embodiments, a passivation film on the back surface of thesubstrate may be thicker than the one on the front surface of thesubstrate, while in some other embodiments, a passivation film on theback surface of the substrate may be thinner than the one on the frontsurface of the substrate. A thickness of the passivation film may vary.In some embodiments, such thickness may be from 30 nm to 300 nm or from50 nm to 250 nm or any value or subrange within these ranges.

The next step is to apply boron emitter metal contacts (511) andphosphorus BSF contacts (512), as shown in FIG. 5M. The formation of themetal contacts is not limited to a particular process. In an embodiment,the metal contacts are formed as described above with reference to FIGS.4N-4R.

There are several methods of depositing the boron-containing paste andphosphorus-containing paste. Screen printing, in particular, isbeneficial for the deposition of the paste since it is commonly used insolar cell manufacturing for the deposition of front and rear metalpastes. For better printing efficiency and performance, the paste ispreferred to be a non-Newtonian or shear-thinning fluid.

Non-Newtonian fluid refers to a fluid that has flow properties that arenot described by a single constant value of viscosity, or resistance toflow. Shear thinning refers to a fluid which has a viscosity thatdecreases with increasing rate of shear. In general, shear thinningbehavior is observed in colloidal suspensions, where the weakhydrostatic and electrostatic interaction between particles and theirsurface groups tends to increase viscosity in non-dynamic force regimes.The addition of a relatively small shear force overcomes the hydrostaticinteraction and thus tends to reduce the viscosity of the fluid.

Consequently, the viscosity of the paste is preferred to be relativelylow at high shear rates in order to pass through a screen pattern, butis also preferred to be relatively high prior to and after deposition(at low or zero shear rates), such that the paste will not run throughthe screen or on the substrate surface respectively.

Boron-containing paste comprises a boron compound; a polymer binder; asolvent, and optionally a ceramic compound. Phosphorus-containing pastecomprises a phosphorus compound; a polymer binder; a solvent; andoptionally a ceramic compound.

The boron compound includes, but not limited to, boron (B), boronnitride (BN), boron oxide (B₂O₃), boric acid (B(OH)₃), boron carbide(B₄C), boron silicide (B₂Si, B₃Si, B₄Si, B₆Si), boron-doped group IVnanoparticles (such as nc-Si:B), aluminum boride (AlB₂), barium boride(BaB₆), calcium boride (CaB₆), cerium boride (CeB₆), chromium boride(CrB), cobalt boride (Co₂B—Co₃B), dysprosium boride (DyB₄, DyB₆), erbiumboride (ErB₄), europium boride (EuB₆), gadolinium boride (GdB₆), hafniumboride (HfB₂), holmium boride (HoB₄), iron boride (Fe₂B), lanthanumboride (LaB₆), lutetium boride (LuB₄), magnesium boride (MgB₂),manganese boride (MnB, MnB₂), molybdenum boride (MoB), neodymium boride(NdB₆), nickel boride (NiB), niobium boride (NbB₂), praseodymium boride(PrB₆), rhenium boride (Re₇B₃), samarium boride (SmB₆, scandium boride(ScB₂), strontium boride (SrB₆), tantalum boride (TaB₂), terbium boride(TbB₆), thulium boride (TmB₄), titanium boride (TiB₂), tungsten boride(WB, W₂B, W₂B₅), vanadium boride (VB₂), ytterbium boride (YbB₆), andzirconium boride (ZrB₂, ZrB₁₂).

The boron compound is between 0.5 and 50 wt % in an embodiment, andbetween 1 and 10 wt % in another embodiment, based on the total weightof the paste.

The phosphorus compound includes, but not limited to, salts of linearphosphoric acids H(O—PO(OH))_(n)OH, where n≧1, and amount of substituentcations X≦n+2; salts of cyclic phosphoric acids (O—PO(OH))_(n), wheren≧3, and amount of substituent cations X≦n, mono- and di-substitutedsalts of phosphorus acid HPO(OH)₂, and salt of hypophosphorus acidH₂PO(OH); and substituent cations X are ammonium, its organicderivatives, and metal cations, preferably Al, Ba, Ca, Ce, Mg, Hf, Ta,Ti, Zr, La.

The phosphorus compound is between 20 and 90 wt % in an embodiment, andbetween 40 and 70 wt % in another embodiment, based on the total weightof the paste.

A ceramic compound may be included in the paste. During the hightemperature diffusion process, boron and phosphorus are allowed todiffuse into the substrate, white the diffusion of ambient materials maybe blocked or substantially reduced by the ceramic material. In anembodiment, the ceramic compound is selected in terms of compatibilitywith the silicon substrate. Some of the oxide materials in contact withsilicon at an elevated temperature may get reduced introducingimpurities into the wafer.

The ceramic compound include, but not limited to, SiN, SiO₂, SiC, TiO₂,Al₂O₃, MgO, CaO, Li₂O, BeO, SrO, Sc₂O₃, Y₂O₃, La₂O₃, CeO₂, Ce₂O₃, Pr₂O₃,Nd₂O₃, Sm₂O₃, EuO, Gd₂O₃, Ta₂O₅, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃,Yb₂O₃, Lu₂O₃, ThO₂, UO₂, ZrO₂ and HfO₂. In an embodiment, the ceramiccompound is selected from the group consisting of silicon oxide (SiO₂)titanium oxide (TiO₂), aluminum oxide (Al₂O₃), magnesium oxide (MgO),tantalum oxide (Ta₂O), and zirconium oxide (ZrO2), and combinationsthereof.

In an alternate configuration, the particle surface of the ceramiccompound may be treated with a ligand or capping agent in order todisperse in a set of solvents and optimize shear thinning behavior. Ingeneral, a capping agent or ligand is a set of atoms or groups of atomsbound to a “central atom” in a polyatomic molecular entity. The cappingagent is selected for some property or function not possessed by theunderlying surface to which it may be attached.

For the selection and modification of the ceramic compound, publiclyavailable paper such as K. J. Hubbard and D. G. Seldom, Thermodynamicstability of binary metal oxides in contact with Silicon, J. Mater.Research, v 11(11), 1996) and prior patent application such as USUtility Patent Publication US20120280183 can be referred to.

The ceramic compound is between 0 and 50 wt % in an embodiment, between3 and 40 wt % in another embodiment, 5 and 30 wt % in still anotherembodiment, based on the total weight of the paste.

A polymer binder may be included in the paste in order to optimizeviscoelastic behavior of the paste for screen printing. The polymerbinder includes, but is not limited to, polyacrylates, polymethacrylate,polyacetals and their derivatives (e.g. methyl, ethyl, butyl),polyvinyls, a cellulose (including its ethers and esters), andcopolymers thereof. Two or more types of the polymer binder can be usedin mixture.

The polymer binder is between 0.5 and 10 wt % in an embodiment, between0.5 and 3 wt % in another embodiment, and between 0.75 and 2 wt % instill another embodiment, based on the total weight of the paste.

The component of the paste can be dispersed in a solvent, such asalcohols (e.g. terpineol), aldehydes, ketones (e.g. cyclohexanone),carboxylic acids, esters, amines, organosiloxanes, halogenatedhydrocarbons, and other hydrocarbon solvents. In addition, the set ofsolvents may be mixed in order to optimize physical characteristics suchas viscosity, density, polarity, etc.

For the purposes of this disclosure and unless otherwise specified, “a”or “an” means “one or more.” All patents, applications, references andpublications cited herein are incorporated by reference in theirentirety to the same extent as if they were individually incorporated byreference.

The invention has been described with reference to various specific andillustrative embodiments. However, it should be understood that manyvariations and modifications may be made while remaining within thespirit and scope of the invention.

EXAMPLES Experiment 1

A set of 156 mm pseudo square, 180 micron thick, n-type, 5 Ohm-cmsilicon wafers were oxidized in a quartz furnace at 1000° C. in anoxidizing ambient for 60 minutes creating an oxide layer ofapproximately 50 nm on both sides of the wafers. The oxidized waferswere screen printed with a phosphorus-containing paste using a firstpattern. Printed wafers were subsequently baked in air at 170° C. forapproximately 1 minute.

Wafers were then loaded into a quartz tube furnace and subjected to atemperature of about 925° C. for 60 minutes in an N₂ ambient. The waferswere then dipped in a dilute hydrofluoric acid and hydrochloric acidsolution to remove the oxide layer. Wafers were cleaned in a hot mixtureof H₂O₂, HCl and water, followed by HF:HCl to remove any residualphosphorus-containing paste and to clean the wafer surface for the nextoxidation step.

Wafers were then oxidized in a quartz furnace at 1000° C. in anoxidizing ambient for 60 minutes creating an oxide layer ofapproximately 50 nm on both sides of the wafers. The oxidized waferswere screen printed with a boron-containing paste using a second patternwherein the first pattern and the second pattern collectively form aninterdigitated pattern.

Printed wafers were subsequently baked in air at 170° C. forapproximately 1 minute. Wafers were then loaded into a quartz tubefurnace and subjected to a temperature of about 925° C. for 60 minutesin an N₂ ambient. The wafers were then dipped in a dilute hydrofluoricacid and hydrochloric acid solution to remove the oxide layer. Waferswere cleaned in a hot mixture of H₂O₂, HCl and water, followed by HF:HClto remove any residual boron-containing paste.

FIG. 6 shows a schematic of the interdigitated pattern where the boronemitter region is indicated as (604), the non-printed region isindicated as (602) and phosphorus BSF region is indicated as (600). Theboron emitter regions (604) were approximately 1600 microns wide. Thenon-printed or gap regions (602) were approximately 150 microns wide.The phosphorus BSF regions (600) were approximately 300 microns wide.Boron and phosphorus SIMS profiles were measured in the center ofregions (600), (602), and (604). An additional SIMS profile was alsomeasured on the far edge of region (604) adjacent to non-printed region(602).

The SIMS profiles measured in the center of the boron-containing pasteprinted area (604) corresponding to the emitter region are shown in FIG.7A with the boron profile indicated as (701) and the phosphorus profileindicated as (702). The SIMS profiles measured in the far edge of theboron printed region (604) adjacent to the non-printed region (602) areshown in FIG. 7B with the boron profile indicated as (703) and thephosphorus profile indicated as (704). It is clear from FIGS. 7A and 7Bthat there are negligible phosphorus signals ((702) and (704))indicating that localized boron doping has been achieved in the boronemitter region.

The SIMS profiles measured in the center of the non-printed region (602)corresponding to the gap are shown in FIG. 7C with the boron profileindicated as (705) and the phosphorus profile indicated as (706).Profiles (705) and (706) show that both boron and phosphorus doping hasbeen suppressed in the non-printed region (602) indicating thatlocalized doping was achieved.

The SIMS profiles measured in the center of the phosphorus-containingpaste printed area (600) corresponding to the BSF region are shown inFIG. 7D with the boron profile indicated as (707) and the phosphorusprofile indicated as (708). It is clear from FIG. 71) that the boronsignal (707) is negligible indicating that localized phosphorus dopingwas achieved in the BSI region (600).

Experiment 2

Monocrystalline 156 mm pseudo-square n-type silicon wafers, 180 umthick, were textured in a KOH solution to form random pyramid texturefor enhanced light trapping. Next, after chemical cleaning, the waferswere loaded in the quartz tube furnace, and they were diffused in theN₂:O₂:POCl₃ ambient at 810° C. for 20 min to form phosphorous dopedlayers at the wafer surfaces. Then, the diffused layer was etched fromthe rear side of the wafer in the mixture of nitric acid, hydrofluoric(HF) acid and water, using silicon nitride as a mask on the front sideof the wafer. Further, after removal of the etch mask in a mixture ofhydrofluoric acid (HF), hydrochloric acid (HCl), and water, the waferswere oxidized in a quartz furnace at 1000° C. in a dry oxidizing ambientfor 120 minutes creating an oxide layer of approximately 80 nm on bothsides of the wafers.

The oxidized wafers were screen printed with a boron-containing paste toform a first pattern of lines (approximately 700 um wide with a pitch of1.2 mm) on the rear side of the wafer. Printed wafers were subsequentlybaked in air at 170° C. for approximately 1 minute.

Wafers were then loaded into a quartz tube furnace and subjected to atemperature of about 950° C. for 60 minutes in an N₂ ambient. The waferswere then dipped in a mixture of HF, HCl and water to remove the oxidelayer. Wafers were further cleaned in a hot mixture of H₂O₂, HCl andwater, followed by HF:HCl to remove any residual boron paste and toclean the wafer surface for the next oxidation step.

Subsequently, the wafers were oxidized in a quartz furnace at 855° C. ina wet oxidizing ambient for 60 minutes creating an oxide layer ofapproximately 50 nm on both sides of the wafers. Then the oxidizedwafers were screen printed with a phosphorous-containing paste using asecond pattern with 300 um wide fingers, wherein the first pattern andthe second pattern collectively form an interdigitated pattern. Printedwafers were subsequently baked in air at 170° C. for approximately 1minute.

Wafers were then loaded into a quartz tube furnace and subjected to atemperature of about 905° C. for 60 minutes in an N₂ ambient. The waferswere then dipped in a mixture of HF, HCl and water to remove the oxidelayer. Wafers were cleaned in a hot mixture of H₂O₂, HCl and water,followed by HF:HCl to remove any residual phosphorous paste.

Further, silicon nitride passivation layers, about 80 nm thick with arefractive index of 2.1, were deposited on the front and the rear sideof the wafers.

Next, metal contacts were deposited on the rear side of the wafers. Thewidths of printed metal fingers were about 100 um and 150 um,respectively. Subsequently the wafers were fired in a belt firingfurnace at a peak temperature of about 800 C. Next, a pattern ofdielectric insulation material was double-printed onto the rear side ofthe wafer, followed by Ag paste busbars, which were aligned to thedielectric pattern. The role of the dielectric is to provide electricalinsulation between the fingers and the busbars of the opposite polarity.Table 1 shows solar cell efficiency results, obtained under simulatedAM1.5G spectrum.

TABLE 1 Measured efficiency from the lot of 13 solar cells. Voc Jsc FFEfficiency Condition (mV) (mA/cm2) (%) (%) Best Cell 644.5 39.92 77.3419.8 Average 642.1 39.79 77.1 19.7 Standard 1.3 0.09 0.24 0.06 Deviation

What is claimed is:
 1. A method for manufacturing an interdigitated back contact solar cell, comprising steps of: (a) providing a doped silicon substrate, the substrate comprising a front, sunward facing, surface and a rear surface; (b) forming a first silicon dioxide layer on the front surface and the rear surface; (c) depositing a boron-containing doping paste on the first silicon dioxide layer of the rear surface in a first pattern, wherein the boron-containing paste comprises a boron compound and a solvent; (d) heating the silicon substrate in a first ambient to a first temperature and for a first time period in order to locally diffuse boron into the rear surface of the silicon substrate, thereby forming a p+ region on the rear surface of the silicon substrate; (e) removing the first silicon dioxide layer from the silicon substrate; (f) forming a second silicon dioxide layer on the front surface and the rear surface; (g) depositing a phosphorus-containing doping paste on the second dioxide layer of the rear surface in a second pattern, wherein the phosphorus-containing doping paste comprising a phosphorus compound and a solvent; (h) heating the silicon substrate in a second ambient to a second temperature and for a second time period in order to locally diffuse phosphorus into the rear surface of the silicon substrate, thereby forming an n+ region on the rear surface of the silicon substrate; and (i) removing the second silicon dioxide layer from the silicon substrate, wherein the first pattern and the second pattern collectively form an interdigitated pattern.
 2. A method of claim 1, wherein the steps (b), (c), (d) and (e) are conducted prior to the steps (f), (g), (h) and (i).
 3. A method of claim 1, wherein the steps (f), (g), (h) and (i) are conducted prior to the steps (b), (c), (d) and (e).
 4. A method of claim 1, wherein the silicon dioxide layers are formed by a method selected from the group consisting of (i) thermal oxidation, (ii) depositing a liquid composition comprising silicon dioxide particles, (iii) chemical vapor deposition, (iv) chemical oxidation, (v) steam oxidation, (vi) printing and oxidizing a composition comprising of silicon particles, and (vii) growing and oxidizing a porous silicon layer.
 5. A method of claim 1, wherein the thickness of the silicon dioxide layers is between 10 nm and 100 nm.
 6. A method of claim 1, wherein a doped layer is formed on the front surface of the silicon substrate prior to the step (b) and wherein the doped layer is doped with the same dopant as the dopant of the silicon substrate.
 7. A method of claim 1, further comprising steps of: (j) forming a passivation layer on both the front surface and the rear surface; and (k) forming a first electrode and a second electrode on the rear surface, wherein the first electrode is in electric contact with the p+ region on the rear surface of the silicon substrate and wherein the second electrode is in electric contact with the n+ region on the rear surface of the silicon substrate.
 8. A method of claim 7, wherein the passivation layer comprises a first passivation layer and a second passivation layer formed on the first passivation layer, wherein the first passivation layer consists of silicon oxide or aluminum oxide and wherein the second passivation layer consists of silicon nitride.
 9. A method of claim 7, wherein the first electrode and the second electrode are formed by screen printing and firing a paste containing silver powder.
 10. A method of claim 7, wherein the step (k) comprises the steps of: (k1) forming the first electrode over the p+ region on the rear surface; (k2) forming the second electrode over the n+ region on the rear surface; (k3) forming a patterned insulator layer over part of the first electrode and part of the second electrode; (k4) forming a first busbar having electrical connection with the first electrode, wherein electrical shunt path between the first busbar and the second electrode is prevented by the insulator layer existing between the first busbar and the second electrode; and (k5) forming a second busbar having electrical connection with the second electrode, wherein electrical shunt path between the second busbar and the first electrode is prevented by the insulator layer existing between the second busbar and the first electrode.
 11. A method of claim 10, wherein the patterned dielectric insulator layer is formed by screen printing a paste comprising a ceramic powder or oxide powders and an organic vehicle over the first and second electrodes.
 12. A method of claim 1, wherein the first temperature is 850-1000° C., wherein the first ambient is nitrogen, oxygen, or mixture thereof, wherein the second temperature is 850-1000° C., and wherein the second ambient is nitrogen, oxygen, or a mixture thereof.
 13. A method for manufacturing an interdigitated back contact solar cell, comprising steps of: (a) providing a doped silicon substrate, the substrate comprising a front, sunward facing, surface and a rear surface; (b) doping the rear surface of the substrate homogeneously with boron in a first pattern, thereby forming a p+ region on the rear surface of the silicon substrate, wherein the first pattern is a blanket pattern; (c) forming a silicon dioxide layer on the front surface and the rear surface; (d) depositing a phosphorus-containing doping paste on the rear surface in a second pattern, wherein the phosphorus-containing doping paste comprising a phosphorus compound and a solvent; (e) heating the silicon substrate in an ambient to a temperature and for a time period in order to locally diffuse phosphorus into the rear surface of the silicon substrate, thereby forming an n+ region on the rear surface of the silicon substrate through the second pattern, wherein the p+ region and the n+ region on the rear surface collectively form an interdigitated pattern; and (f) removing the second silicon dioxide layer from the silicon substrate.
 14. A method of claim 13, wherein the rear surface of the substrate is doped with boron by one selected from the method consisting of deposition of boron-containing doping paste followed by a thermal drive-in step, ion implantation, BBr₃ diffusion, BCl₃ diffusion, and BSG deposition using chemical vapor deposition followed by a thermal drive-in step. 